Many reduced-bit-rate audio coding techniques are “block-based” in that the encoding includes processing that divides each of the one or more audio signals being encoded into time blocks and updates at least some of the side information associated with the encoded audio no more frequently than the block rate. As a result, the audio information, when decoded, has a temporal envelope resolution limited by the block rate. Consequently, the detailed structure of the decoded audio signals over time is not preserved for time periods smaller than the granularity of the coding technique (typically in the range of 8 to 50 milliseconds per block).
Such block-based audio coding techniques include not only well-established perceptual coding techniques known as AC-3, AAC, and various forms of MPEG in which discrete channels generally are preserved through the encoding/decoding process, but also recently-introduced limited bit rate coding techniques, sometimes referred to as “Binaural Cue Coding” and “Parametric Stereo Coding,” in which multiple input channels are downmixed to and upmixed from a single channel through the encoding/decoding process. Details of such coding systems are contained in various documents, including those cited below under the heading “Incorporation by Reference.” As a consequence of the use of a single channel in such coding systems, the reconstructed output signals are, necessarily, amplitude scaled versions of each other—for a particular block, the various output signals necessarily have substantially the same fine envelope structure.
Although all block-based audio coding techniques may benefit from an improved temporal envelope resolution of their decoded audio signals, the need for such improvement is particularly great in block-based coding techniques that do not preserve discrete channels throughout the encoding/decoding process. Certain types of input signals, such as applause, for example, are particularly problematic for such systems, causing the reproduced perceived spatial image to narrow or collapse.